This match will be tricky to predict because just as you have said it's a derby match even though Manchester United has not been in the best of forms when it comes to big teams especially there city rivals united seams to up their game, so this game will be very interesting to watch as both teams will be fighting to get all three points.
Is very hard to predict the outcome of this match, you know if it were before when manchester city is still in a good form I would've possible choose them as the favorite team to win this match considering the home advantage, but is obvious that manchester city is going through the same challenge with manchester united and this would balance the strength of both teams so if I'm to predict the score I think I will go for goal goal or Draw that's the both teams to score our they end up with a score of draw. I'm sure that these prediction is more suitable for this game, than going for direct win.